Method for the manufacturing of baked ore pellets

ABSTRACT

A method of making baked ore pellets of prescribed composition and dimensional characteristics in a flow sequence which comprises a. grinding the ore, b. mixing the ground ore with a binding agent, c. moistening the mixture, d. forming the moist mixture into green pellets, and e. thereafter baking such green pellets in a baking zone, is improved by F. DIVERTING FROM THE FLOW SEQUENCE GREEN ORE PELLETS FORMED IN STEP D. WHICH DEVIATE FROM THE PRESCRIBED COMPOSITIONAL CHARACTERISTICS, G. DIVERTING FROM THE FLOW SEQUENCE GREEN PELLETS AND BROKEN GREEN PELLETS FORMED IN STEP D. WHICH DEVIATE FROM THE PRESCRIBED DIMENSIONAL CHARACTERISTICS, H. DIVERTING FROM THE FLOW SEQUENCE BAKED PELLETS AND BROKEN BAKED PELLETS FORMED IN STEP E. WHICH DEVIATE FROM THE PRESCRIBED DIMENSIONAL CHARACTERISTICS, I. FORMING A COMBINED FLOW OF THE GREEN AND BAKED MATERIALS DIVERTED IN STEPS F., G. AND H., AND J. MIXING THE COMBINED FLOW WITH FRESH ORE TO CONSTITUTE THE ORE BEING GROUND IN STEP A.

United States Patent Van Latenstein BAKED ORE PELLETS [75] inventor:

Netherlands [73] Assignee:

METHOD FOR THE MANUFACTURING 0F Anthonie Van Latenstein, Sanipoori,

Koninklijke Nederlandsche Hoogovens en Staaliabriken N.V., ljmuiden, Netherlands Filed:

Aug. 31, 1971 Aug. 30, 1971 Appl. No.: 176,038

Foreign Application Priority Data Netherlands 7112875 US. Cl 75/3, 75/5, 432/21,

References Cited UNITED STATES PATENTS Hyde 263/53 R Ahlmann... Rea 263/53 R ORE YARD HOPPERS DRYING GRINDING BINDING AGENT MIXERS FORMING Primary Examiner-John J. Camby Assistant Examiner-Henry C. Yuen Attorney-J. Y. Houghton [5 7] ABSTRACT A method of making baked ore pellets of prescribed composition and dimensional characteristics in a flow sequence which comprises a. grinding the ore, b. mixing the ground ore with a binding agent, c. moistening the mixture, (1. forming the moist mixture into green pellets, and e. thereafter baking such green pellets in a baking zone, is improved by f. diverting from the flow sequence green ore pellets formed in step d. which deviate from the prescribed compositional characteristics,

g. diverting from the flow sequence green pellets and broken green pellets formed in step d. which deviate from the prescribed dimensional characteristics,

h. diverting from the flow sequence baked pellets and broken baked pellets formed in step e. which deviate from the prescribed dimensional characteristics,

. forming a combined flow of the green and baked materials diverted in steps 11, g. and h., and j.

mixing the combined flow with fresh ore to constitute the ore being ground in step a.

4 Claims, 2 Drawing Figures DEVIATING SCREEN BAKING SCREEN ORE PELLETS BROKEN ORE PELLETS V FINAL PRODUCT HOPPERS METERING OVERFLOW PATENTEB I082] I975 BINDING AGENT SHEEI 1 0F 2 ORE YARD HOPPERS DRYING ZPB'L GRINDING MIXERS I FORM'NG DEVIATING I ORE PELLETS 2 SCREEN BROKEN oRE PELLETS BAKING 6 3 4 SCREEN HOPPERS METERING FINAL PRODUCT QVERFLOW INVENTOR BY M 7% ATTORNEY METHOD FOR THE MANUFACTURING OF BAKED ORE PELLETS This invention relates to a method for the manufacturing of baked ore pellets, in which the ore in consecutive treatments is first ground, is thereafter mixed with a binder and is thereafter moistened, after which the mixture thus obtained is formed into pellets which are baked in a baking zone.

As a charging material for blast furnaces the usual lump ore and sinter is being more and more replaced by baked ore pellets. It has appeared that thereby the capacity of a blast furnace may be increased, while moreover a saving in the quantity of coke to be used is obtainable. The best results are obtained if the ore pellets have a high quality as to uniformity of shape and dimensions and are to be considerable extent free from ore dust and rough particles of ore broken during the manufacture of the pellets. Thus it is usual, after burning or baking of the ore pellets, to screen the product thus obtained. In this way the particles not being good pellets are separated and may be ground again in a separate mill to give fine material which may be fed back to the stored ore for feeding into the process of making the pellets. It has also been suggested to feed such fractions of particles screened off as not being good pellets to an ore sintering plant if this is also available.

During the entire manufacturing process of the ore pellets waste is also liberated in other regions. This is particularly the case after forming the ore pellets from the moistened mixture of ore dust or flour with binder. This forming of the ore pellets may be done in rotating drums or on rotating dishes. In both systems part of the ore pellets formed will have deviating dimensions and part of the ore pellets will have insufficient coherence, so that it is impossible to convey them to the baking plant without damage. In view thereof screening takes place before the baking furnace is reached, so that pellets being too small and broken unbaked ore pellets are separated from the good unbaked, so-called green pellets. In many cases this screening will take place by feeding the flow of pellets over a roller grid with rotating eccentric rollers which acts as a sieve for all fractions having dimensions differing from those of the good ore pellets.

The screening after forming of the pellets has as an advantage that the permeability of the bed of ore pellets in the baking furnace and the uniformity of this permeability are promoted. This not only accelerates the process of baking and thereafter cooling of the ore pellets, but moreover the control of this process is simplified because the deviations in the permeability of the bed of pellets are smaller.

The return flow from the screening plant, which screens the material after the forming of the pellets, is often led to the feed side of this forming plant.

It is possible that-a further return flow occurs during the heating-up or during the adjusting period towards optimum operation of the plant. The same is true for the situation which occurs during a switching to another quality of the pellets to be manufactured. In such a situation there will be a time period in which ore pellets are formed which have a quality differing both from the pellets made before and from the pellets to be made after the transition. This deviating quality may be due to a deviating composition, e.g. a deviating moisture content, but also to a deviating shape or density of the material. In all these cases it is undesirable that the formed wet" or green ore pellets are fed to the baking furnace. Therefore in many cases a possibility is provided to guide the product of the forming plant temporarily out of the system, e.g. to storing beds for the fresh ore.

It has appeared that the usual methods to feed such fractions separated out of the main flow of ore either back into the process or away to some other destination show disadvantages. A main disadvantage is seen in the fact that the return feed to different zones in the process makes it necessary to have different separated return flow lines with all the conveyors etc. necessary therefor. This may give considerable complications in the lay-out of the entire plant and in control thereof in operation. Installing a separate mill for the return flow from a point downstream of the baking furnace means an expensive additional capital investment, and the feeding of this material to a sintetring plant is liable to considerable restrictions in applicability and is often even totally impossible.

It has been suggested before to feed the material screened out after the forming of the pellets back to the forming plant. However, variations in the quantity of this screened-out material will give variations in amount and quality of the flow of material to the forming plant, which gives instabilities and difficulties in the entire process. In particular differences in the flow of material from the forming plant may also give rise to a variation in the changing of material to the baking fur nace, which in its turn will harmfully affect the baking process itself and thus the quality of the burned ore pellets.

It has also been suggested before to feed the formed unburned or green ore pellets of deviating quality back to the ore yard, where the fresh ore is stored.

In view of the above the present invention aims at a considerable simplification as to all these possible return flows of material. In this respect the invention aims at also obtaining more stability in the process and more uniformity of the baked final product.

In this respect the invention is characterized in that in a method as given in the preamble all the flows of material separated from the main straightthrough flow of the ore through the plant are united and combined and are thereupon fed to the grinding plant. This makes it possible to use a simple conveying system for conveying all the return flows. As the return material is collected in storage hoppers of the grinding plant it is thus possible to remove all instability in the process of the forming plant, in which the green pellets are formed.

It is possible that the method of the invention as given above also will allow a restriction of the complexity of the plant if a so-called wet grinding system is applied. However, in this case the danger exists that the varying quantity of material will cause a disturbance in the grinding process. This can be avoided and the best results can be obtained if according to a preferred realization of the method of the invention the grinding or milling takes place according to a so-called closed dry grinding system, in which also before or during the beginning of the grinding operation an additional drying of the flow of material takes place. E.g. such drying may take place in a so called windsifter, before the supply of the material to the grinding plant, or in a drying chamber which is part of a grinding mill.

If this supply takes place in the windsifter the material already ground will be dried and it is thus possible to separate it easily immediately. If the material is fed to the grinding mill itself having a drying chamber or the like, this material will also immediately be discharged after drying by the gas flow maintained in the mill. Thus this material to be separated will not itself be subjected to grinding and thus the control of the grinding mills is not disturbed.

In this respect a closed grinding system is a system in which the material to be ground is supplied in a flow of material which circulates through a grinding device and a separator, and in which the finest ground fractions are separated from the circulating material in the separator and are removed from the circulating flow of material. In many cases it will be possible to use a gas cyclone as a separator in such a plant.

It has already been remarked that it is possible to apply a wet grinding system. In that case it will be possible to feed both the return flow already baked (baked return) and the return flow of wet unbaked material (wet return) to a plant for wet grinding. As a separator in the closed grinding system it would in this case be possible to use a set of hydrocyclones or liquid cyclones, in which the wet return would be supplied to the slurry which is past through said cyclones. Thus the already fine fractions in the wet return will be kept outside the flow of useful material through the grinding mills.

The return flow of baked material will, however, with such a method constitute a considerable problem. Feeding this material back into the grinding mills will cause serious difficulties. This is so because in these mills using a wet grinding process the effect of the grinding on the return flow of baked material is very low. Thus there will be an accumulation of baked returned material in the mills, which will cause frequent disturbances of the process, mainly as a result of clogging and over-filling thereof. It is remarked that it is possible that considerable variations occur in the quantity of material screened off behind the baking plant. This could entrain that the return flow of this material will cause rather considerable variations in quality in the flow of the main material and product,-such variations being of short duration. In has appeared that according to the invention this disadvantage can be avoided by first collecting the screened off baked material to be returned in storage hoppers and thereupon metering it from said hoppers into the return flow to the grinding plant in a constant proportion to the fresh" ore flow to said grinding plant.

This metering system can be adjusted in such a way that in case of an unexpected large quantity of screened off baked material part of this material will be discharged from said hoppers as an overflow e.g. to a sintering machine. As a rule, however, this last measure will not have to be taken.

The present invention will now be explained in more detail with reference to the enclosed drawings, giving a preferred embodiment of the method according to the invention, and in which:

FIG. 1 gives a flow sheet of this preferred method, and.

FIG. 2 gives more or less diagrammatically part of the apparatus used in this method.

In FIG. 1 the manufacturing line has been shown from the ore to and inclusive the baked ore pellets as a final product, ready for feeding to the blast furnace, as a number of production steps in series connection,

shown from top to bottom. Ore is fed from the ore yard to storage hoppers, from which it is fed to a grinding plant. This plant operates according to the principle of dry grinding, so that it also includes a drying zone. This drying zone may fully or in part be united structurally with the grinding mills. Thereupon a binding agent is added to the ground ore and mixed thereto in mixers. A binding agent which is often used as a binder for fine ore is a sticking type of clay known as bentonite. Mainly during mixing the mixture is also moistened by water, so that a kneadable mass is obtained. This mass is fed to the forming plant for the pellets, in which in forming drums or on forming dishes so-called wet or green ore pellets are formed. These green pellets are fed to the baking plant over a screen, formed as a moving roller grid, and in said baking plant they are formed to a bed of pellets on a screen, which is continuously moved through the baking furnace. After passing the screen behind this machine the burned ore pellets can be discharged to a storage space as the final product. It is remarked that the described flow sheet includes grinding mills, mixers, forming apparatus, sieves or screens and baking furnaces, which are generally known in the art of making ore pellets. So it will not be necessary to give a more detailed description of such apparatus.

In the flow sheet three return flows are indicated by reference numerals 1, 2 and 3, relating respectively to green ore pellets of deviating type, dimensions or composition, to broken green ore pellets and to material derived from the baking plant and screened off from the pellets after baking. As remarked before the return flow l of the deviating green ore pellets will essentially occur during starting up and adjusting to optimum conditions of the forming process in a pelletizing device or during switching to another product. The product of the return flow which is derived from the screen behind the baking machine is first fed to intermediary hoppers. From these hoppers this material is thereupon conveyed as a return flow 4, accurately metered by metering apparatus.

The return flows 1, 2 and 4 are united to a single return flow 5, which is fed to the storage hoppers into which the ore from the ore yard is first fed at the beginning of the process.

The metering of the return flow 4 takes place dependent on the flow of material from the ore yard to said storage hoppers, for which a signal 6 is used. In this way it is possible to obtain that the so-called baked return is fed back into the flow through the plant in a quantity, which has a constant proportion to the quantity of the ore fed to the manufacturing plant. This minimizes the influence of this baked return on the quality of the final product. Line 7 indicates an overflow, eg to a sintering machine, if the quantity of baked return is unexpectedly large.

In FIG. 2 part of the device used in realizing the method according to the invention is shown diagrammatically. Reference numeral 8 shows a storage hopper, in which an ore flow 13 is supplied from an ore yard. From this hopper an ore flow 14 goes to a drying chamber 10 which constitutes part of a ball mill 9, which is partly filled with balls 11. The ground product 15 is supplied to an air cyclone 12, in which separation takes place between fine and coarse material. The fine material 17 is fed to the mixers, which are not shown, the coarse material is returned in a flow 16 into the drying chamber 10. The drying chamber is heated by blowing combustion gas 18 therein, which is made in a separate combustion chamber not shown by burning gaseous fuel therein.

What is claimed is:

1. An improvement in a method of manufacturing burned or baked ore pellets of prescribed compositional and dimensional characteristics, said method being of the type which comprises, in a flow sequence:

a. grinding the ore,

b. mixing the ground ore with a binding agent,

c. moistening the mixture,

d. forming the moist mixture into green pellets, and

e. thereafter baking such green pellets in a baking zone,

the improvement comprising, in combination in said method:

f. diverting from the flow sequence green ore pellets formed in step d. which deviate from the prescribed compositional characteristics,

g. diverting from the flow sequence green pellets and broken green pellets formed in step d. which deviate from the prescribed dimensional characteristics,

h. diverting from the flow sequence baked pellets and broken baked pellets formed in step e. which deviate from the prescribed dimensional characteristics, l

i. forming a combined flow of the green and baked materials diverted in steps f., g. and h., and

j. mixing the combined flow with fresh ore to constitute the ore being ground in step a.

2. An improvement as claimed in claim 1, wherein k. the grinding step a. is affected by a closed dry grinding process in which before or during the grinding operation a drying is affected of the ore being ground.

3. An improvement is claimed in claim 2 wherein the amount of baked material combined with green material in step i. is metered to be constantly proportional to the amount of fresh ore with which the combined flow is mixed in step j.

4. An improvement is claimed in claim 1, wherein the amount of baked material combined with green materialin step i. is metered to be constantly proportional to the amount of fresh ore with which the combined flow is mixed in step j.

UNITED STATES PATENT @FFECE CERTIFICATE 0F CORECTION Patent No. 3,753, 1 Dated August 21, 973

Ifiventoz-(s) Anthpnie Van Latenstein It is certified that errer appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

In the heading, change itemfiQ] to reed --[3 )7 Foreign Application Prigr ity Data Au 31, 1970 Netherlands 70.12875-- Signed ana sealed this 19th day of February 1971;.

(SEAL) Attes't:

EDWARD M.FLETCHER,JR. M HALL DANN l mu Attesting Officer Commissioner f Patent FORM po'wso (169) uscoMM-Dc 60376-P69 U. 54 GOVERNMENT PRINTING OFFICE: "I 0-lll-J3l 

2. An improvement as claimed in claim 1, wherein k. the grinding step a. is affected by a closed dry grinding process in which before or during the grinding operation a drying is affected of the ore being ground.
 3. An improvement is claimed in claim 2 wherein the amount of baked material combined with green material in step i. is metered to be constantly proportional to the amount of fresh ore with which the combined flow is mixed in step j.
 4. An improvement is claimed in claim 1, wherein the amount of baked material combined with green material in step i. is metered to be constantly proportional to the amount of fresh ore with which the combined flow is mixed in step j. 